1. FIELD OF THE INVENTION
The present invention relates to computer systems and in particular to a device interconnection bus for use in interconnecting components of a computer system.
2. RELATED APPLICATIONS
This application is related to U.S. patent application Ser. No. 07/994,983, entitled "Method and Apparatus for Arbitrating on an Acyclic Directed Graph," which is assigned to the assignee of the present invention.
3. DESCRIPTION OF RELATED ART
Device interconnection busses are commonly employed to connect individual components or devices of a computer system, or a group of networked computers, to each other to allow communication between the components. Although the interconnection bus is primarily provided to facilitate communication of data, commands and other signals between computer devices, conventional busses also provide a transmission line for distributing power to at least some of the devices connected to the bus.
Generally, a device connected to a bus either has its own power-supply or it requires power drawn from the bus. A device power-supply may be a battery power-supply or a power-supply capable of connecting into a wall electrical socket. Examples of computer devices which typically include a power-supply are printers, monitors, scanners, large disk drives, large speaker/amplifiers, video equipment, central processing units (CPU) and portable computers such as lap top or notebook computers. Portable computers typically have both battery-powered capability, as well as, AC power transformer capability for drawing power from an AC electrical wall socket. Examples of devices which typically lack a power-supply and therefor must draw power from the bus include a mouse, microphone, hand-held scanner, network access bridge, small disk drive, keyboard, cable extender and the like.
A sufficient amount of power must be supplied to the bus to power the bus itself and to power those devices which require power from the bus. A power-supply is provided which is capable of outputting a sufficient amount of power to the bus to power all devices connected to the bus that may require bus supplied power. Conventionally, the amount of power required is determined by assuming that all devices may be in operation at one time, and that such devices will be drawing maximum power. In other words, the computer system is configured by providing a power-supply sufficient to power all devices simultaneously at their maximum power levels. However, in practice not all devices operate simultaneously and, when operated, devices do not always consume the maximum amount of power which the device is capable of consuming. Hence, a power-supply of a conventional bus is capable of providing far more power than nominally required. This unnecessary power-supply excess adds costs to the system which may not be justified if not all devices do not operate simultaneously. In a conventional system, a fairly modest power-supply is provided, and the number of devices that can be connected to the bus is simply limited accordingly. Although this solution successfully prevents an overload of the bus caused by too many devices drawing too much power, the solution unnecessarily limits the capabilities of the bus. One of the primarily advantages of a bus interconnection is the flexibility of allowing a variety of devices to be interconnected and interchanged conveniently. This flexibility is lost if the total number of devices allowed for connection to the bus is unduly limited, or if the power-supply must be upgraded to allow an increase in the number of devices.